A practical power generation system utilizing electrostatic induction by an electret material is disclosed in PLTs 1 to 6 etc. “Electrostatic induction” is the phenomenon where if making a charged object approach a conductor, a charge of the opposite polarity to the charged object is induced. A power generation system utilizing the phenomenon of electrostatic induction is comprised of a “film holding a charge” (below, referred to as an “electrically charged film”) and a counter electrode. It utilizes this phenomenon and makes the two move relatively to induce a charge to be taken out.
FIG. 1 is an explanatory view schematically explaining the principle of power generation utilizing the phenomenon of electrostatic induction. In FIG. 1, the counter electrode side is made to move, but the electrically charged film side may also be made to move.
If taking as an example the case of an electret material, an electret has a dielectric material into which a charge is injected and is a type of electrically charged film generating an electrostatic field semipermanently. In power generation by an electret, as will be seen in FIG. 1, the electrostatic field formed by the electret 3 causes an induced charge to be generated at a counter electrode (electrode) 2. If making the area of overlap of the electret 3 and counter electrode 2 change (due to vibration etc.), it is possible to cause the generation of an AC current at an outside electrical circuit E. This power generation by an electret is advantageous in the point that the structure is relatively simple and a higher output is obtained in the low frequency region compared with power generation by electromagnetic induction. This has therefore been the focus of attention in recent years as so-called “energy harvesting”.
FIGS. 18(a) to (c) are views showing an outline of counter electrodes and electrically charged films of PLT 1 as prior art. FIGS. 19(a) to (c) are views explaining the counter electrodes and electrically charged films of PLT 1. FIG. 20 are explanatory views for explaining the Coulomb forces acting on the areas of the overlapping parts of electrically charged films 3 and first electrodes A and second electrodes NA of the counter electrodes 2 of FIGS. 19.
PLT 1 discloses a power generation system utilizing electrostatic induction in which electrically charged films and counter electrodes engage in reciprocating periodic rotation. As one embodiment of this prior art, unlike the schematic view of FIG. 1, an embodiment is shown in which electrodes taken out as output are formed only on the counter board. As shown in FIG. 18(a), the bottom surface of the rotating member 4 is formed with electrically charged films 3. The fixed side counter board 1 is alternately formed with a plurality of first electrodes A and a plurality of second electrodes NA. Reference notation 13 is a spiral spring set between the rotating member 4 and a shaft 8. Reference notation 10 is a rotating weight (rotor).
The plurality of first electrodes A are electrically connected with each other. An interconnect 90A is used to take out the generated power. The plurality of second electrodes NA are also electrically connected with each other. An interconnect 90N is used to take out the generated power at a rectifying circuit 92. The voltages output from the first electrodes A and the second electrodes NA are alternately generated by electrostatic induction, so a waveform of an alternating current offset by a half cycle phase is output. The interconnect 90A connected to the first electrodes A on the counter board 1 and the interconnect 90N connected to the second electrodes NA are connected to the rectifying circuit 92 using diodes 91. The rectified power is connected to a storing member 93 using a capacitor or secondary cell etc. The power charged at the storing member 93 drives a later stage electronic device circuit 94. In the embodiment of PLT 1 where the electrodes are formed only on the counter board, current can be taken out from the fixed side counter board, so the design is convenient (there is no need to take out current from the rotating member).
At the bottom surface of the rotating member 4, electrically charged films 3 such as shown in FIG. 19(a) are formed. The parts of the rotating member 4 between electrically charged films 3 and other electrically charged films 3 are formed with holes. On the other hand, as shown in FIG. 19(b), as the counter electrodes 2 on the counter board 1 fixed at a position facing the electrically charged films 3, first electrodes A and second electrodes NA are alternately formed. The first electrodes A and the second electrodes NA are respectively connected with each other. The interconnects 90A and 90N taken out from the first electrodes A and the second electrodes NA are connected to the rectifying circuit 92 using the diodes 91 and, furthermore, are connected to the storing member 93 using a capacitor or secondary cell etc.
The voltages output from the first electrodes A and the second electrodes NA are alternately generated by electrostatic induction due to the rotation of the rotating member 4, so a waveform of an alternating current is output. FIG. 18(c) shows the arrangement of the electrically charged films 3 and counter electrodes 90A and 90N when viewed from the circumferential side surface of the rotating member 4. The counter electrodes 90A and 90N are alternately arranged. The electrically charged films are arranged at the same intervals as those between the counter electrodes 90A or 90N. If the rotating member 4 is made to rotate, the electrically charged films and the counter electrode face each other by any of the positional relationships of FIGS. 18(b) and (c). That is, as shown in FIG. 18(b), if a first electrode A faces an electrically charged film 3, the first electrode A attracts a plus charge and current flows in one direction. Simultaneously, at a second electrode NA at a position not facing an electrically charged film 3, the attracted plus charge is dissipated and current flows in the opposite direction to the above direction. Next, the rotating member 4 rotates resulting in FIG. 18(c). FIGS. 18(c) and (b) are repeated. Specifically, a first electrode A at a position facing an electrically charged film 3 formed at the rotating member 4 and a second electrode NA at a position not facing an electrically charged film 3 become opposite in polarity, so the interconnects 90A and 90N are connected to different input terminals of the rectifying circuit 92. The alternating current output from the power generation system is converted to direct current by the rectifying circuit 92 and charged into the storing member 93. If electric power sufficient for driving an electronic device circuit 94 connected at a later stage is charged, the later stage electronic device circuit 94 can be driven.
FIG. 19(c) shows the arrangement of electrically charged films 3 and counter electrodes A and NA and the effect of the Coulomb forces when viewing the rotating member 4 with the electrically charged films of FIG. 19(a) and the counter electrodes of FIG. 19(b) facing each other as seen from the circumferential side surface. A “Coulomb force” is the force of attraction acting between charges of opposite polarities. The force of attraction becomes greater the larger the charges. Due to the arrangement of the first electrodes A and second electrodes NA of FIG. 19(b), as shown in FIG. 19(c), a Coulomb force acts between an electrically charged film 3 and an electrode A (or second electrode). Due to the component F of its direction of movement, a sawtooth shaped holding torque such as shown in FIG. 20(b) ends up acting on the rotating member. Note that, the first electrodes A and the second electrodes NA of FIG. 20(a) are inherently fan shaped, but for facilitating the explanation are shown as rectangular shapes.
When the rotating member 4 stops, it stops at a position where the holding torque of the rotating member 4 becomes maximum, that is, the position where the overlapping areas of the electrically charged films 3 and electrodes A or NA become maximum. Therefore, at the time of start of rotation of the rotating member 4, the rotating member 4 will not rotate and external vibration cannot be converted to power even if applied unless a rotating force greater than the peak value of the holding torque is applied. Accordingly, if a sawtooth shaped holding torque such as shown in FIG. 20(b) ends up acting on the rotating member, the extremely high peak value of the holding torque ends up raising the threshold value of the initial torque of the rotating member 4 and there is a limit to further improvement of the efficiency of energy conversion from external vibration of environmental vibration. Further, looking at the continuity of rotation and vibration of the rotating member 4 obtained from environmental vibration as well, peak values of the holding torque repeatedly occur making it impossible to obtain rotation or vibration continuing for longer periods of time.
PLT 2 also discloses a rotary type power generation system utilizing electrostatic induction where electret films and counter electrodes engage in reciprocating periodic rotation. Electret films are formed at the inner surface of a rotating member and counter electrodes are formed at a fixed side counter board facing it. The electret films of the rotating side and the counter electrodes of the fixed side are used as electrodes to take out current. In PLT 2, current has to be taken out from the rotating side electrets as well, so this is troublesome.
In the prior arts of PLTs 1 and 2, in each case, the electrically charged films and the counter electrodes of the facing board are made the same shapes and power is generated by relative movement of the positional relationship of the electrically charged films and counter electrodes. In the case of such a structure, in electret power generation, Coulomb forces Q are generated between the electrically charged films and counter electrodes, so the initial torque required for the rotating member to start to move has to be a torque of the Coulomb forces or more. Further, even when the torque transmitted to the rotating member is gone and the rotating member rotates by inertia, rotation stops when the inertia becomes the Coulomb forces or less. For this reason, to improve the power generation efficiency of electret power generation, it is necessary to reduce the Coulomb forces generated between the electrically charged films and counter electrodes. PLT 3 discloses a type where the electret films and counter electrodes engage in translational motion, but again a similar problem arises.
As opposed to the prior arts of the above PLTs 1 to 3, in each of the electrostatic induction type power generation systems using electret films of PLTs 4 and 5, a movable board engaging in reciprocating motion is sandwiched between a top fixed board and bottom fixed board. The top and bottom surfaces of the movable board are respectively formed with electret films. Counter electrodes facing the electret films on the top surface of the movable board are provided on the top fixed board, while counter electrodes facing the electret films on the bottom surface of the movable board are provided on the bottom fixed board. The phases of the pitch of the counter electrodes and electret films in the direction of movement are offset from each other between the top part and bottom part of the movable board to reduce the Coulomb forces, reduce the initial torque at the time of power generation, and improve the power generation efficiency. However, the dual top-bottom surface type of PLTs 4 and 5 has the following problem.
In the dual top-bottom surface type, the Coulomb forces can be cancelled out only when the top surface electrically charged films and the bottom surface electrically charged films are equal in amounts of electric charge. The movable board has to be positioned at an accurate intermediate position between the top fixed board and bottom fixed board to balance the Coulomb forces. For this reason, control of the positional precision of the movable board is difficult. On top of this, the amounts of electric charge mainly depend on the thicknesses of the electrically charged films. In the production process, not only do the film thicknesses end up varying, but also, since corona discharge is used for charging, the amounts of electric charge also often vary. Therefore, in the dual top-bottom surface type, making the amounts of electric charge of the top and bottom electrically charged films equal has been a considerably difficult problem.
Furthermore, to utilize the top and bottom surfaces of the movable board, there has to be a certain thickness in the vertical direction between the top fixed board and movable board and between the movable board and the bottom fixed board. There was therefore the problem that the power generation device became greater in thickness.